Data storage libraries conventionally include a controller subject to the command of a host computer, a plurality of media elements, such as optical discs and magnetic tape cartridges, an array of slots for storing the media elements, a drive for performing read/write operations on the media elements, and a robotic transport for transporting the media elements between the slots and the drive. The library controller regulates the storage of the media elements, the transport of the media elements between the slots and the drive, and the read/write operations.
The robotic transport typically includes a so-called picker device or assembly designed to grasp the media element in a slot, transport the media element to the drive, insert the media element into a drive, and release and deposit the media element in the drive, as well as to perform a reverse process by which it grasps the media element within the drive, transports the media element to a slot, inserts the media element in the slot, and releases and deposits the media element in the slot.
The picker assemblies typically include a pair of arms or jaws moveable toward each other for the purpose of gripping, clasping, or clamping the media element and away from each other for the purpose releasing the media element. Typically both arms move under motor actuation and control.
Knowing the width of the media elements used within a data storage library, the motor actuating the movement of the picker arms may be controlled so as to move the arms to an open position to receive the media element and then to close the picker arms to a specified distance apart essentially equal to the width of the media element. Typically, a polyurethane pad or another type of resilient member is placed on the inwardly facing side of each arm and is adapted to abut the lateral sides of the media element to help absorb and distribute the compressive force of the arms with the media element clasped between the arms. Further, the resilient member may be provided with a non-slip or relatively frictional surface that is adapted to help maintain the media element in a compressively clamped condition between the arms. The resilient members on each arm may initially contact the media element, and then the arms are further squeezed together to provide a clamping force, thereby ensuring that the element will be maintained in a position between the jaws as the media element is moved and transported within the data storage library. Such an operation of squeezing the arms through the resilient members will create an initial resistive force and then an increase in resistive force until a predetermined gripping, clasping, or clamping force is attained.
The speed of operation of a data storage library is an important quality, and therefore increasing the speed of operation of a data storage library is a desirable objective. Thus, it is desirable to increase the speed of operation of the picker assembly in grasping and releasing media elements. The type of motor for actuation of the arms should act quickly and with precision. One such motor is a stepper motor. Some of the advantages of a stepper motor are that it is relatively inexpensive, it is capable of precise positioning and repeatability of movement, it possesses an excellent response to starting/stopping/reversing commands, it is very reliable and possesses a relatively long life, and it has a wide range of rotational speeds that can be realized. However, stepper motors also have potentially severe drawbacks. For example, stepper motors have a natural frequency of operation, a so-called self-resonant frequency. If the excitation frequency matches the self-resonant frequency, then a condition of ringing or vibration can become severe and can cause the motor to lose torque. Also, if the stepper motor experiences an increased load, the torque of the motor may slip, resulting in a loss of synchronization and even a reversal of rotation. These undesirable conditions can severely affect the operation of the stepper motor and can require a re-calibration of the stepper motor. The occurrence of these disadvantageous conditions of a stepper motor when operating a picker can shut down the operation of the data storage library.
The present invention reduces the possibility that such undesirable conditions will occur. A stepper motor as used to move picker arms may encounter unexpected loads. For example, sometimes the media elements are deposited in the slots in an angled or skewed orientation such that, when the arms move inwardly to clasp the media element the arms encounter a resistive force when the arms first contact the media element. The arms will experience a continued, usually increasing, resistive force as the arms continue to move together until the media element is operably oriented within and clasped between the arms. If a stepper motor is used to move the arms inward and the resistive force is encountered, the stepper motor will experience a sudden load which may cause slippage, especially if the stepper motor is being operated at a relatively high speed.
The present invention was developed in an attempt to utilize the advantages of a stepper motor, including its high speed operation, while at the same time reducing or eliminating any slippage due to the resistive force as the arms clamp the media element.